عنوان مقاله [English]
This study was conducted with the aim of investigating the effect of layer coating of nano polyurethane and nanoclay on improving the properties of cold-storage white top kraft paper used in the packaging industry. For this purpose, white top kraft paper with a base weight of 131 g.m-2 was prepared and tested. To cover the surface of the paper, nano-polyurethane was first applied by spraying with a nozzle in an amount of about 15 g.m-2 on the surface of the white top paper. In the second stage, to improve the performance of the coating material of the first stage, the surfaces of cold-storage white top kraft paper were covered with nanoclay by a laboratory coating machine. In this step, the coating material was coated with the amount of 27 g.m-2 on the upper surface of white kraft paper. After being coated and restrained, the samples were dried at room temperature for one day to stabilize the coating material on its surface. Then, the samples were placed inside the freezer for 2 and 4 months and their properties were measured. Before the tests, the control and coated samples were placed in standard environmental conditions (at a temperature of 20 οC and relative humidity 65%). The results showed that the coating (control) increased the thickness, smoothness of the surface, and the resistance against the ring crush of the paper in the direction of the machine and reduced the water absorption, brightness and other mechanical properties such as the tensile strength index machine direction (MD) and cross direction (CD), burst strength index, tear strength index (MD and CD), and ring crush test in the cross direction in frozen coating samples with increasing thickness and surface smoothness; a decrease in other mechanical properties was observed. The samples coated twice showed very few pores. The thickness and surface smoothness of white coated kraft paper showed an increase of 30.8% and 37.7%, respectively, compared to the control sample. Water absorption, burst strength index, tear strength index (MD and CD), tensile strength index (MD and CD), ring crush test (MD and CD), and the brightness of the double-coated white kraft paper compared to the control sample, respectively showed a decrease of 151, 87.4, 15, 13, 104.2, 103.2, 67.1, 82.3 and 11.8%. According to the obtained results, it is recommended that papers such as liquid packaging cardboard, juice and milk packets, which have many applications in food packaging, should be tested with these coatings with different times.